My Case:
IDOCS are posted to webMethods TN. This kicks off process model and details are sent out to customer in the particular format required (cXML/EDI).
For some requests, the 3rd step in process model is failing (failure to establish correlation). This is random behaviour, happens only to some requests.

Can anybody help me in finding reason for this random behaviour?

Error message I got:
Correlation was unsuccessful for process with ConversationID=0000000018708582 and ProcessInstanceID = 9a3ae5400de011dea3d6e84df847d188 com.wm.lang.flow.FlowException: Correlation was unsuccessful for process with ConversationID=0000000018708582 and ProcessInstanceID = 9a3ae5400de011dea3d6e84df847d188 at com.wm.lang.flow.FlowExit.getFailure(FlowExit.java:250) at com.wm.lang.flow.FlowState.willExit(FlowState.java:268) at com.wm.lang.flow.FlowSequence.invoke(FlowSequence.java:196) at com.wm.lang.flow.FlowRoot.invoke(FlowRoot.java:258) at com.wm.lang.flow.FlowState.invokeNode(FlowState.java:581) at com.wm.lang.flow.FlowState.step